buying pre-made trusses and rafters

I need to make a roof for a new porch / front extension, which is 4.5M x 1.8M. The roof is a lean-to design with a hip end on one end. (compliant with building regs)

Is the 'done thing' to buy the wall plates, trusses etc pre-made from a supplier who'll build them to my dimensions? Is there much to be saved by attempting to make it all myself? I like the idea of buying it as a kit to be honest!

something of that size you could form yourself 4x2 timber and a measure twice cut once rule dont think you would get trusses for a porch 4.5m wide by 1.8m spar length much cheaper to do it yourself or get a joiner in someroofes would do it to included in roof price

Are there some fairly simple rules to follow in terms of building regs - 600mm gap between trusses, use 4x2" timber, and whatever zinc hangers you find in the local merchants?

Something I've not mentioned is that I'd like the space to be usable, for a small amount of storage, so will fit a hatch and probably lay chipboard over the insulation. Will the building inspector want to look at my truss cross section and want me to prove, somehow, that it can deal with snow/wind loads?

I agree re: the amount of space. I've not actually sketched out the likely cross sections involved, so I've no idea how realistic that aim is. I don't expect to be able to get a person up there, but the kind of space you can stick a head into an keep the odd box of decorations etc.